This invention relates generally to switching systems and methods, and more particularly to a system and method for enabling synchronization between an interface card and a switch.
A crossbar-based switching fabric is synchronous by nature. The switch includes a scheduler for controlling matches on the crossbar. The scheduler receives requests for connections through the crossbar, makes the matches through a grant/accept process, and aligns the crossbar for that match. The crossbar will stay aligned for one cell time. The cells that are to be switched through the crossbar must therefore be present at the crossbar at the precise time that the crossbar is aligned.
The switch card contains the crossbar that may be centrally located in the switching system. A conventional physical layout is shown in FIG. 1. As illustrated, switch 100 includes a chassis with a plurality of vertical line cards 111-118. The chassis is typically 19″ wide. Depending on the location of switch card 120, the distance from any line card 111-118 to switch card 120 could be as short as 4″ or as far as 19″ direct line distance. The electrical trace distance, however, will be significantly longer as the signals are routed between line cards 111-118 and switch card 120 through a bus supported by a backplane in the chassis. As would be appreciated, the distance from any line card 111-118 to switch card 120 will typically be different.
The distance issue can be further complicated if the line card is located in a chassis separate from the switch card. In this arrangement, the interconnect link between the line cards and the switch card may be supported by an optical connection. A system diagram of an optical-interconnect implementation is illustrated in the example embodiment of FIG. 2.
As illustrated, optical-interconnect implementation 200 includes line cards 210 and 220. Line cards 210 and 220 further include opto-electrical converters (OECs) 212 and 222, respectively. Line cards 210 and 220 are interconnected to switch card 230 via optical links 240 and 250, respectively. Optical links 240 and 250 are terminated on switch card 230 through OEC 232. In various implementations, optical links 240 and 250 can span lengths of three feet to 1800 feet.
The potential of a significant length disparity between two separate links interconnected on the same switch card raises significant synchronization issues. These synchronization issues can be attributed at least in part to the transmission time of a bit on an optical link 240, 250. In general, the flight time can be on the order of one nanosecond per foot of optics. If the fiber length is 300 meters, then we could have 1.5 microseconds of delay for the bit stream. This 1.5 microseconds of delay can translate into a delay length that spans multiple cells. What is needed therefore is an efficient mechanism for accounting for the delay time on the link.